1. Field
This invention relates to the field of the crystalline materials to be used in fabrication of the disk, active mirror or other similar laser devices. More particularly, it pertains to the use of laser devices where an appropriately oriented optical substrate is diffusion-bonded to a lasing medium. Such implementation allows the resulting laser apparatus to be thermally loaded to much higher levels (compared to the levels usually used when the unbonded laser medium is directly attached to a heat sink) without risk of catastrophic failure of the device.
2. Description of Related Art
In prior art for face-pumped laser disks, which are typically pumped on one side and cooled on the other side, the laser medium is simply mounted directly to the metal heat sink, typically copper with an indium interface. Such a design is widely used and is well know to those skilled in the art.
In case of the design mentioned above, fracture will occur when the thermally induced tensile stress at one face reaches the surface tensile strength of the medium. For certain laser materials with poor thermomechanical properties, for instance, yttrium vanadate, such failure occurs even when the level of thermal loading is quite low. Thermal loads as low as a few watts per centimeter could be sufficient to cause the failure.
Typical failure modes for thermally loaded laser media occur when the surface stress is under tension and exceeds the tensile strength of the material. The failure normally happens when the temperature profile of the media is such that the surface is cooler than the bulk interior portion of the media. For the vast majority of media (those with a positive linear thermal expansion coefficient), this type of temperature profile creates a tensile stress at the surface as opposed to the compressive stress in the bulk interior. That is, the surface expands less than the bulk and, therefore, the bulk interior puts the surface under tension (i.e., the bulk interior is pulling the surface apart).
One way to address the thermal problem has been to face cool both sides of the laser medium with liquid or gas. This method is well known to those skilled in the art. The disadvantage of this method of cooling is that the laser optical path must transit through the cooling medium and, therefore, aberrations are imposed on the beam, thus degrading the beam quality of the laser output.
Another way to address the thermal problem is to diffusion-bond a dissimilar material of higher thermal conductivity and hardness to the weaker laser material. For example, neodymium-doped yttrium vanadate (Nd:YVO4), has been bonded to Al2O3 (sapphire) but the extreme difference in linear thermal expansion coefficients between the two materials causes the YVO4 material to cleave spontaneously, thereby limiting the ultimate size of the YVO4. In particular, Onyx Optics, Inc. of Dublin, Calif., has diffusion bonded Nd:YVO4 to sapphire with some limited success.
However, to date, the bonded interface size has been limited to several millimeters. This constraint is likely to exist due to the substantial difference in thermal expansion coefficients between the two materials, as well known to those skilled in the art.
Therefore, methods and devices known in the prior art have significant disadvantages and drawbacks, since it is not possible to scale the laser medium of previous designs to significantly larger sizes (for higher power devices) without the cleavage failure and, due to stress-induced cleavage failure, the prior art devices cannot be operated at increased level of pumping. Increased pumping is desirable to achieve, since it leads to increased power outputs.
Therefore, there exists a need in the art for a laser apparatus in which the laser medium can be scaled up to significantly larger sizes without the cleavage failure of previous designs.